Graphite as a lubricating agent in fault zones: An insight from low‐ to high‐velocity friction experiments on a mixed graphite‐quartz gouge

Abstract

[1] Graphite is a very low friction material, often enriched within fault zones due to mechanical or chemical processes. The effects of weak minerals on the strength of faults have been examined by friction experiments on bimineralic mixtures. However, previous experiments were conducted with limited shear strains, even though applied shear strains and textural developments had already been signaled as significant factors in the weakening of faults. We therefore conducted large-displacement, low-to high-velocity friction experiments with graphite-quartz gouges, to determine how much graphite is needed to reduce frictional strength, and to examine how textures contribute to the strength reduction of a mature fault at various slip rates. We found that the coefficients of friction of the gouges decrease nonlinearly with increasing graphite fraction for any given shear strain and slip rate, decreasing first with 5-20 vol% graphite, then reaching similar frictional levels to pure graphite with 30-50 vol% graphite. The nonlinear weakening trends can be fitted by sigmoidal curves. The weakening with 10-30 vol% graphite is associated with zones of slip-localization and the development of a graphite-lubricated penetrative slip surface(s). With increasing shear strain, the relationship between strength and graphite fraction evolves abruptly from an early gentle curve to a sigmoidal curve, and the frictional strength drops significantly even with small amounts of graphite (~10 vol%). Our results highlight the importance of shear strain and textural developments on weak faults, not only with respect to graphite, but also other fault lubricants such as the phyllosilicates.Citation: Oohashi, K., T. Hirose, and T. Shimamoto (2013), Graphite as a lubricating agent in fault zones: An insight from low-to high-velocity friction experiments on a mixed graphite-quartz gouge,

Strain-induced amorphization of graphite in fault zones of the Hidaka metamorphic belt, Hokkaido, Japan

“…These observations suggest that the aggregates were formed by the accumulation of fragmented graphite, and reoriented by the strong shear deformation. Such aggregates of deformed graphite resemble the products observed on the shear surfaces of high-velocity, rock friction experiments (Oohashi et al, 2011(Oohashi et al, , 2013.…”

Composition, Alteration, and Texture of Fault-Related Rocks from Safod Core and Surface Outcrop Analogs: Evidence for Deformation Processes and Fluid-Rock Interactions

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A rotary-shear low to high-velocity friction apparatus in Beijing to study rock friction at plate to seismic slip rates

“…This apparatus is often called the second high-velocity machine, but its slip rate capability covers a very wide range, 10 -10 m/s (about 3 mm/a) to almost 10 m/s, and normal stresses to about 20 MPa for cylindrical specimens of 25 mm in outer diameter. It was used for gouge experiments Togo et al 2011), shear-induced graphitization and graphite behaviors (Oohashi et al 2011(Oohashi et al , 2013, energetics of gouge deformation Sawai et al 2012), temperature suppression due to dehydration (Brantut et al 2011), possible powder lubrication , and initiation processes of an earthquake-induced landslide (Togo et al 2014).…”